Cyclindrical cavity for household ovens

ABSTRACT

This disclosure consists of a cylindrical cavity making up the interior of gas or electric ovens for domestic use allowing the ovens that use this cylindrical cavity to optimize heat energy transmitted by convection allowing them to heat up more quickly and in less time than square section ovens, saving up to 29% of the energy consumed during the cooking process.

TECHNICAL FIELD

This following is related to the field of electrical appliances and gas appliances; concretely related to the component of the oven; specifically with a cylindrical cavity, for use in manufacturing domestic use ovens destined for cooking food in the home. This cylindrical cavity will be a part of full ovens and electrical appliances, including gas ovens, electrical ovens, flush mounted ovens and toaster ovens for domestic use.

BACKGROUND

Currently, the appliances industry uses the concept of ovens with a square-section cavity, consisting of a rigid structure to which heat is applied using gas burners or electric resistances.

The fact that the section conventionally used in ovens is square means that the heat generated by the heat source isn't distributed uniformly within the oven's volume, and low temperature zones are formed within the vortexes of the same.

SUMMARY

An oven using a cylindrical cavity for domestic ovens improves this situation considerably; it is more efficient and consumes less energy. This can be concluded by applying the proven formulas and methods for this purpose.

According to Incropera (1), heat transfer is produced in ovens by convection, due to the global movement of the molecules between liquid and gaseous states. The rate of heat transmitted from the surface of one hot body to another by convection is given by:

q=hAΔT ¹ INCROPERA, Frank & DeWITT, David. Fundamentos de transferencia de calor. 4th ed. Prentice Hall: Mexico. 1999. 912 p.

Where:

q=heat transfer rate

h: film heat transfer coefficient, which depends on geometry, with a circular cavity being more efficient as it improves heat flow. When the corner is a right angle the heat doesn't flow in the same way. In ovens that use square cavities, the “turbulence” generated by the right angles increases the heat of the chassis, not of the food.

A: Is the surface area. There are no dead areas when the circular cavity is used and the oven's volume is more effective, since in ovens using square surfaces, more heat is needed to fill the entire volume.

ΔT: The temperature difference between the heat source and the fluid.

The Nusselt Number (Nu) is an a-dimensional number (a constant reached after prolonged research) that measures the increase in heat transfer from a surface over which a fluid flows.

In a circular tube characterized by a uniform superficial heat flow and fully developed laminar conditions, the Nusselt number is a constant, independent of ReD (the Reynolds number), Pr (the Prandt number) and the axial position.

${Nu}_{D} = {\frac{hD}{k} = \text{4,36}}$

This formula is used to calculate the value of “h” in the initially described formula, used to obtain the rate of heat transfer from one hot body to another by convection.

Where h is the film heat transfer coefficient, D is the diameter of the tube and k the thermal conductivity of the fluid. Many engineering applications imply convection transport in non-circular tubes. In a non-circular tube, convection coefficients vary around the periphery, approaching zero in the corners. Therefore, when using a circular cavity, this coefficient is an average over the entire perimeter, and no temperature variations exist on the inside.

For laminar flow, the use of circular tube correlations is less precise, particularly with cross sections characterized by sharp corners. In these cases, the Nusselt number corresponding to fully developed conditions can be obtained from the following table, based on the solutions to differential equations for flow momentum and energy along the different tube cross sections.

The Nusselt numbers tabulated for uniform surface heat flow assume a constant flow in the axial direction (flow), but a constant temperature around the perimeter of any cross section. [Incropera, 1999, p448]

TABLE 1 Nusselt numbers and friction factors for fully developed laminar flows in tubes with different cross sections: Nu_(D) = hDh/k Cross Section b/a Uniform Q''s Uniform Ts Circular — 4.36 3.66 Square 1 3.61 2.98 Rectangular 1.43 3.73 3.08 Rectangular 2.0 4.12 3.39 Rectangular 3.0 4.79 3.96 Rectangular 4.0 5.33 4.44 Rectangular 8.0 6.49 5.60 Source: Incropera, 1999

Nusselt had defined number “4,36” but for this number to valid, we need to use the same conditions he used, i.e., a square oven. What Icropera did was adjust the table and the measurement to circular geometric shapes, i.e., 3,66.

Description of the Test for Verifying an Oven's Energy Efficiency

The previous information is useful for theoretically calculating an oven's energy efficiency. The following type of assay has the goal of proving the theoretical calculations experimentally.

The oven is operated under normal operating conditions and an aluminum mass is located on the center or wherever the supports (shelves) allow. The assay ends when the mass has achieved an increase of 50K. The gas supply is cut off and the final values for the block's temperature are recorded waiting for its temperature to stabilize. Finally the gas volume is totaled. The model used for the efficiency calculation is the following:

$\eta = \frac{m \times {Cp} \times \Delta \; T}{{Vn} \times {PCS}}$

Where:

m: Mass of the aluminum block.

Cp: specific heat of the aluminum =0.836 Kj/kg° K.

ΔT: Temperature increase for the aluminum block: 50K for this assay.

Vn: measured gas volume carried to standard reference conditions.

PCS: higher calorific power of the gas at standard reference conditions.

General conditions for the assays.

Gas type Natural (PCS = 10.29 Kw/m³) Assay pressure [mbar]  20 ± 1 Atmospheric Pressure [mbar] 851 ± 3 Mass (load) of aluminum. 3.85 Kg and 8.35 Kg. Assay room temperature [° C.] 25° C. ± 2° C. ΔT [K] 50

Current Applications for Ovens with Cylindrical Cavities

The concept of ovens with cylindrical cavities is currently used in industrial, metallurgical, ceramic applications, among others. This concept is also used in laboratories. These uses mean that these ovens are extremely different artifacts to the cylindrical cavity for domestic ovens, which provide important energy savings, the possibility of being assembled from pieces, which make them easier to transport, and with much lower costs and materials than other ovens with this type of cavity, which make it more appropriate for domestic use.

The cylindrical cavity for domestic ovens allows domestic ovens to improve their energy efficiency and heat food more rapidly.

The a cylindrical cavity for domestic ovens facilitates the movement of hot air allowing it to reach all the points inside the oven, it helps the heat accumulated on the walls to be reflected by radiation in a uniform manner to every area where the sources with the food to be cooked may be located.

In ovens using a cylindrical cavity for domestic ovens, the heat source is located at the chamber's focal point. The net effect of this arrangement can be approximated to the creation of a uniform heating effect inside the cylinder; i.e., one can assume that the resulting flow of heat to the fluid q″s is a constant along the length of the circumference. 2[p 440].

Although a circular oven's useful volume is 10% less than the standard shape, this doesn't affect its normal use, taking into account that the maximum width of a circular oven (38 cm) allows introducing a wider tray than a standard oven, which can only take up to 35 cm trays.

The best shape for an oven is cylindrical. There are many reasons for this. When one analyses heat flux data (flow per unit of area W/m2) presented in Table 1, calculated by Incropera, and also when one compares theoretical heat transfer values for rectangular ovens, one finds that:

When the oven's base to height ratio is 1,43, the heat flux is 3,73.

According to this, heat transfer in circular ovens is more efficient, and equals 15%.

Besides, in the aforementioned experimental tests we can observe that energy consumption efficiency is 29% compared to the square section oven in FIG. 3.

BRIEF DESCRIPTION

FIG. 1:

Figure showing the cylindrical cavity for domestic ovens in perspective.

FIG. 2:

Showing an “exploded” graphic allowing identification of the components of the cylindrical cavity for domestic ovens.

FIG. 3:

Schematic of the side panels. ² INCROPERA, Frank & DeWITT, David. Fundamentos de transferencia de calor. 4th ed. Prentice Hall: Mexico. 1999. 912 p.

FIG. 4:

Front Chassis Schematic

FIG. 5:

Schematic of the Rear Cover

DETAILED DESCRIPTION

The cylindrical cavity for domestic ovens consists of a metal chassis assembled using several panels called the left side panel (FIG. 2, item D); the right side panel (FIG. 2, item C); the front chassis (FIG. 2, Item A); the back cover (FIG. 2, Item B); the base or diffusor (FIG. 2, Item E), and the burner protection (FIG. 2, Item F).

Initially the metal sheet advances along a conveyer belt until reaching a programmed limit. There it is cut according to the size of each panel, resulting in square shapes. Each shape is then introduced into a sheet-shearing machine where it is crop cut, and then the shape is pushed in as far as it will go and the cutting of the part's shape continues.

The cut panels then move to the stuffing machines, die shears and folding machines, which, with the help of other tools, begin the process of transforming the initial blanks.

The, the left (FIG. 2, Item D) and right (FIG. 2, Item C) side panels are put in a press, introducing the part into the die right to the end in order to obtain the rail-shaped veins for sliding in the grill used to support heating utensils; the piece is curved and the ends folded to obtain tabs, which are the perforated in order to help with assembly.

Then, the part that will make up the chassis front (FIG. 4) is placed in the stuffing machine or die cutter. It is introduced in the die guides and then the machine is activated, thus obtaining the final part. The part that will be used for the back cover (FIG. 5) is pressed, and the body of it is perforated according to a similar procedure as that used for the front chassis.

In order to build the diffusor base (FIG. 2, Item E) which will allow directing the heat provided by the burner located in the lower part. Its body is perforated with a die cutter introducing the part in the die cutter to the end. The diffusor base is pressed and then folded at the ends to obtain tabs that will then serve for assembly.

The parts described are then submitted to a self-cleaning process via a process of immersion and the application of enamel using a pneumatic pistol to the parts that make up the chassis' panels. This is applied to the chassis front, side panels, chassis back, rear cover and diffusor base. Then they are passed through a drying oven.

Once the parts that will make up the cylindrical cavity for domestic ovens have been manufactured, self-cleaned and enameled, the artifact is assembled. The left and right panels are fixed with 5 rivets and, once they are assembled they are fixed to the back cover with 9 rivets. Then the front chassis is attached to the side panels with 9 rivets and finally the diffusor base is fixed to the chasis with 10 rivets. This finally configures the cylindrical cavity for domestic ovens. 

1-6. (canceled)
 7. A cylindrical cavity for domestic ovens comprising: a cylindrical shaped cavity with an internal diameter between 38 centimeters and 90 centimeters, wherein the cylindrical shaped cavity includes at least one rail on a lateral wall of the cylindrical shaped cavity; a rear cover with a flush circular section that copies a form of the cylindrical shaped cavity and allows access to an interior of the cylindrical shaped cavity; and a heat diffuser located in a lower part of the cylindrical shaped cavity; wherein the cylindrical shaped cavity, the rear cover, and the heat diffuser are comprised of metal with a heat resistant finish.
 8. The cylindrical cavity for domestic ovens of claim 7, further comprising: a plurality of curved side with rails, wherein a result of a pressing process that serves to support a plurality of shelves, located at a same distance related to a center of the circumference of an oven section to permit a use of a single shelf and a fold in the joins to form a tab with a plurality of holes which allows an assembly.
 9. The cylindrical cavity for domestic ovens of claim 7, wherein the cylindrical shaped cavity includes a plurality of extension in at least one of an upper section or a lower section to increase a volume of the cylindrical shaped cavity.
 10. The cylindrical cavity for domestic ovens of claim 7, further comprising: a rear cover with a flush circular section copying the cavities form and which facilitates its rear assembly; and a frontal frame with an orifice which allows access to the interior of the cylindrical shaped cavity, and which serves as supports for fixing the cylindrical shaped cavity.
 11. The cylindrical cavity for domestic ovens of claim 7, wherein an inner heat diffuser is assembled on a base of the cylindrical shaped cavity. 